Technical Field
The present invention relates to a distance measuring technique using laser light.
Background Art
Techniques for measuring distances using laser light are publicly known. One such technique is disclosed in Japanese Unexamined Patent Application Laid-Open No. 2003-090881, for example.
According to this technique, laser light that is modulated by a high frequency, as a distance measuring light, is emitted to an object, light reflected from the object is detected, and then a distance to the object is calculated from a phase difference between the emitted light and the detected light. At this time, use of a modulated laser light having a higher frequency enables detection of a smaller phase difference, thereby increasing resolution of the measured distance.
In a case of performing surveys of roads or ground surfaces at a long distance, since laser light is obliquely incident on an object, the laser light is reflected from portions of the object, of which distances are different from each other, and multiple components of the reflected light are detected. The multiple components have different phases, and therefore, the waves of the multiple components interfere with each other due to phase differences, and the intensity of the detected light tends to be low. The intensity greatly decreases as the modulation frequency of the distance measuring light increases. Accordingly, a measurable distance should be shortened.